Here is a short list of useful web accessibility tools.
Graybit – http://graybit.com/main.php
GrayBit is an online accessibility testing tool designed to visually convert a full-color web page into a grayscale rendition for the purpose of visually testing the page’s perceived contrast.
Sitescore – http://www.silktide.com/tools/sitescore
SiteScore is a free tool which rates how well designed, popular and accessible your website is. [Update – They already discontinued the free sitescore edition so the link is already dead. – Jojo]
Accessibility Color Wheel – http://gmazzocato.altervista.org/colorwheel/wheel.php
This handy tool is useful to test the accessibility pairing between background color and text. Choose a foreground color by pointing the mouse over the wheel or the grey stripe and click. Then click the “Background” button and choose a background color the same way. If OK becomes visible down here the color pair is good for accessibility. Otherwise change one color or both selecting foreground and background by the left buttons. Ok in the single textboxes means the contrast/difference in brightness is good.